1. Field of the Invention
The present invention relates to a semipermeable membrane made of a special polymer.
2. Description of the Prior Art
As a reverse osmosis membrane heretofore used for the desalination of sea water and salt water and for the separation of various inorganic salts and organic compounds from solutions, there is known the so-called Loeb-type membrane made of hydrous acetyl cellulose and having an asymmetric structure including a dense and compact surface skin layer and a porous substrate layer. The Loeb-type membrane can be formed into a plate-like product, a spiral coil, a tubular product or a hollow fiber. The Loeb membrane has a sufficiently high capacity for practical use. However, this membrane is disadvantageous because it is readily hydrolyzed by an acid or alkali and/or it is readily decomposed by microorganisms. Accordingly, when the membrane is placed in practical use, the pH of the solution to be treated should be adjusted to from 3 to 7 so that the acetyl cellulose will not be hydrolyzed. Further, when the membrane is not in use, it must be stored in an aqueous solution of a fungicidal or bacteriostatic chemical.
As another example of a reverse osmosis membrane that is in practical use, there can be mentioned a reverse osmosis membrane in the form of a hollow fiber, which is made from an aromatic polyamide. This membrane is disadvantageous because it is readily decomposed by a very small amount of chlorine. Therefore, chlorine must be removed from the solution that is to be brought in contact with this type of membrane.
We have discovered a reverse osmosis semipermeable membrane possessing the advantages of the Loeb membrane, but which eliminates the foregoing-described defects of the known membranes.
Since reverse osmosis is performed using a feed liquid which is under a high pressure, in order to maintain stable the properties of the semipermeable membrane used for the reverse osmosis, it is necessary that compaction of the semipermeable membrane should not occur in the presence of water under a high pressure. Accordingly, a polymer suitable for making the membrane should be stiff enough to resist high pressure in the presence of water and it should contain hydroxyl groups necessary for attaining a sufficient water permeation speed. For example, a commercially available phenoxy resin manufactured by Union Carbide Corporation and having the formula: ##STR2## is very stiff because benzene nuclei are present in the main chain of the polymer. Further, since the hydroxyl groups are present as side chains, this resin has a hydrophilic property.
We have examined the suitability of this phenoxy resin as a material for making a reverse osmosis membrane. As a result, it was found that a phenoxy resin of the foregoing formula possesses an inferior hydrophilic property and the required water permeation speed of a reverse osmosis membrane cannot be attained therewith. Therefore, the phenoxy resin of the foregoing formula, per se, is not suitable as a material for making a reverse osmosis semipermeable membrane. We have discovered, however, that when a semipermeable membrane is made of a polymer obtained by partially replacing the hydroxyl groups of the phenoxy resin of the foregoing formula by at least one substituent selected from sulfoalkyl ether groups, sulfoaryl ether groups, sulfoaralkyl ether groups and salts thereof with an alkali metal, ammonium or nitrogen-containing basic organic compound, the semipermeable membrane has an excellent semipermeable characteristic and the foregoing disadvantages of the conventional acetyl cellulose and aromatic polyamide membranes are overcome.
However, a membrane composed of a polymer formed by partially replacing the hydroxyl groups of the phenoxy resin of the above-formula, with a sulfoalkyl ether group, a sulfoaryl ether group and/or a sulfoaralkyl ether group is disadvantageous in that the reduction of the water permeability of this membrane with the passing of time is conspicuous in comparison with the conventional acetyl cellulose membrane and, hence, this membrane is inferior to the conventional acetyl cellulose membrane with respect to its resistance to compaction under pressure and its creep resistance when used for a long time.
We have discovered a semipermeable membrane which is free from these defects. We have found that a polymer obtained by partially converting the hydroxyl groups of the phenoxy resin of the above-mentioned formula to a sulfuric acid group or a salt thereof with an alkali metal, ammonia or a nitrogen-containing basic organic compound, has an excellent water-permeating property. Moreover, the disadvantages of the conventional acetyl cellulose and aromatic polyamide membranes, and the membranes of the above-mentioned polymers obtained by partially replacing the hydroxyl groups by a sulfoalkyl ether group, a sulfoaryl ether group and/or a sulfoaralkyl ether group, are completely eliminated in a semipermeable membrane made of this polymer. This polymer is unexpectedly superior as a membrane-forming material in comparison with the conventional acetyl cellulose and aromatic polyamides, and the above-mentioned modified phenoxy resin. We have now completed the present invention based on these findings.
More specifically, in accordance with the present invention, there is provided a semipermeable membrane made of a water-insoluble, cross-linked, membrane-forming polymer, hereinafter referred to as "polymer (II)", which is obtained by partially converting the hydroxyl groups of a polymer having the following formula, hereinafter referred to as "polymer (I)": ##STR3## wherein R and R' are halogen, nitro, methyl or ethyl, X is a divalent group selected from methylene, ethylene, isopropylidene, ether (--O--), carbonyl (--CO--), sulfide (--S--), sulfoxide (--SO--) and sulfone (--SO.sub.2 --),
l and m are integers of from 0 to 4, p is 0 or 1, and PA1 n is an integer of from 100 to 1000,
to sulfuric acid groups or salts thereof with an alkali metal, ammonium or a nitrogen-containing basic organic compound, and cross-linking the "polymer (II)".
The term "semipermeable membrane", used in the specification and claims, means a membrane having a selective permeability, which can be used for such membrane separation processes as reverse osmosis, ultra-filtration, dialysis and electrodialysis.
The semipermeable membrane of the present invention will now be described in detail.
The starting polymer (I) that is used for preparing the material polymer (II) used to make the semipermeable membrane of the present invention is a polymer obtained by reacting a dihydric phenol with epichlorohydrin (West German Patent Publication No. 1,545,071) or by reacting a phenoxy resin formed by reaction between a dihydric phenol and epichlorohydrin, with a dihydric phenol (British Pat. No. 980,509). The starting polymer has the formula (I): ##STR4##
In the formula (I), R and R' each are halogen, particularly chloro or bromo, nitro, methyl or ethyl, and l and m are integers of from 0 to 4. When the number of the substituents R or R' is two or more, these substituents can be the same or different. X is methylene, ethylene, isopropylidene, ether (--O--), carbonyl (--CO--), sulfide (--S--), sulfoxide (--SO--) or sulfone (--SO.sub.2 --), and p is 0 or 1 and n is an integer of from 100 to 1000.
Examples of suitable dihydric phenols are bisphenols such as bis(4-hydroxyphenyl)-methane, bis(4-hydroxy-3-methylphenyl)-methane, bis(4-hydroxy-3,5-dichlorophenyl)-methane, bis(4-hydroxyphenyl)-ketone, bis(4-hydroxydiphenyl)-sulfide, bis(4-hydroxyphenyl)-sulfone, 4,4-dihydroxy-phenyl ether, 1,2-bis(4-hydroxyphenyl)-ethane, 2,2-bis-(4-hydroxyphenyl)-propane, 2,2-bis(4-hydroxy-3-methylphenyl)-propane, 2,2-bis(4-hydroxy-3-chlorophenyl)-propane, bis(4-hydroxyphenyl)-phenylmethane, bis(4-hydroxyphenyl)-diphenylmethane, bis(4-hydroxyphenyl)-4'-methylphenylmethane, 1,1-bis(4-hydroxyphenyl)-2,2,2-trichloroethane, bis(4-hydroxyphenyl)-(4'-chlorophenyl)-methane, 1,1-bis(4-hydroxyphenyl)-cyclohexane, bis(4-hydroxyphenyl)-cyclohexylmethane, 4,4'-dihydroxydiphenyl and 2,2'-dihydroxydiphenyl. These bisphenols can be used singly or in the form of mixtures of two or more of them.
The polymer (II) used to make the semipermeable membrane of the present invention can be prepared by partially converting the hydroxyl groups of the starting polymer (I) having the above formula (I), to a sulfuric acid group, a salt thereof with an alkali metal, ammonium or a nitrogen-containing basic organic compound, or a mixture thereof. The substituents formed by such conversion are represented by the following formulae: ##STR5## wherein M is an alkali metal such as Na, K or Li, and A is ammonia or a nitrogen-containing basic organic compound such as aliphatic primary amines such as methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, sec-butylamine, tertbutylamine, pentylamine, isopentylamine, hexylamine, 2-ethylhexylamine and laurylamine; aliphatic secondary amines such as dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, N-methylethylamine and N-ethylisobutylamine; tertiary amines such as trimethylamine, triethylamine, N,N-dimethylpropylamine, tributylamine and N-ethyl-N-methylbutylamine; mono-, di- and tri-ethanolamine; diethylaminoethanol; urea; .beta.-dimethylaminopropionitrile; aliphatic quaternary ammonium compounds such as tetramethylammonium and tetraethylammonium; alicyclic amine compounds such as cyclohexylamine, N,N-dimethylcyclohexylamine, and dicyclohexylamine; aromatic primary amines such as aniline, o-toluidine, m-toluidine, p-toluidine, o-ethylaniline, m-ethyl-aniline, p-ethylaniline, p-isopropylaniline and p-t-butylaniline; aromatic secondary amines such as N-methylaniline, N-ethylaniline and N,N-dimethylaniline; aromatic quaternary ammonium compounds such as trimethylphenyl ammonium and ethyldimethylphenyl ammonium; aralkyl amines such as benzylamine and .alpha.-methylbenzylamine; nitrogen-containing hetrocyclic compounds such as pyrrole, 1-methylpyrrole, indole, pyridine, .alpha.-picoline, .beta.-picoline, .gamma.-picoline, 2-ethylpyridine, quinoline, piperadine, 1-methylpiperidine, 2-methylpiperidine, 1,8-diazabicyclo[5.4.0]undecene-7 and morpholine.
The conversion is generally accomplished by (1) dissolving the polymer (I) in a solvent, such as tetrahydrofuran, dioxane or N,N-dimethylformamide, (2) adding to the solution, at a temperature lower than room temperature, chlorosulfonic acid or anhydrous sulfuric acid in an amount of from 15 to 100 mole %, based on the hydroxyl groups of the starting polymer (I) and (3) conducting the reaction for 1 to 3 hours. After completion of the reaction, the reaction mixture is added dropwise into water or a non-solvent, such as an alcohol, whereby to precipitate the resulting polymer, wherein the hydroxyl groups of the starting polymer (I) have been transformed to ##STR6## groups. If a hydroxide of the above-mentioned alkali metal, or ammonia or a nitrogen-containing basic organic compound as mentioned above is added to the above reaction mixture in an amount of from 2.5 to 5 moles, per mole of anhydrous sulfuric acid or chlorosulfonic acid used for the reaction, the substituted sulfuric acid group is converted to an alkali metal salt, or ammonium salt or a quaternary ammonium salt of the nitrogen-containing basic organic compound. The recovered polymer is washed with water and is dried, preferably at a temperature below 50.degree. C. The sulfated phenoxy resin is readily hydrolyzed when the terminal group is a sulfuric acid group (--OSO.sub.3 H), and its stability is low when it is allowed to stand. However, if the terminal sulfuric acid group is converted to a salt, the resin is stabilized. Therefore, it is preferred that the sulfuric acid group is converted to an alkali metal salt, or ammonium salt or a quaternary salt of a nitrogen-containing basic organic compound.
The starting polymer (I) can be identified by NMR analysis and IR analysis. Confirmation of the introduction of substituents present in the polymer (II) of the present invention and determination of the quantity of the introduced substituents are performed by sulfur analysis of the sulfuric acid groups, NMR analysis of the substituted methine groups or the organic compound forming the salt with the sulfuric acid group or by neutralizing titration.
The thus-obtained polymer (II) is a novel macromolecular compound which, so far as is known, is not disclosed in any literature reference.
The process for preparing a semipermeable membrane from the thus-obtained polymer (II) will now be described.
The casting process is most preferred for forming a membrane. The membrane of the present invention can be uniform or homogeneous in cross-section or it can be as asymmetric (skinned) membrane having an improved water-permeating property, which asymmetric membrane comprises a dense and compact surface skin layer and a porous supporting layer, like a Loeb membrane. The membrane of the former (homogeneous) type can be obtained by dissolving the polymer in a single solvent, casting the solution onto a substrate having a smooth surface, such as a glass sheet, a metal plate, a sheet of a synthetic resin inert to the solvent or a porous sheet, and then gradually evaporating the solvent in a vessel covered with a filter paper or the like. The membrane of the asymmetric type is obtained by dissolving the polymer in a single solvent or a mixed solvent comprising at least two solvents differing in their boiling points, casting the solution onto a substrate such as the substrates mentioned above, removing a part of the solvent by evaporation and treating the cast layer in a coagulating bath. As the solvent, there can be mentioned those as listed in the following Table 1.
Those solvents can be used singly or in the form of a mixed solvent comprising two or more of them.
A low-boiling-point organic compound having a good compatibility with such solvent and a non-solvent of the type used for the coagulating bath can be added to the polymer solution prior to casting, provided that the solubility of the polymer is not lowered. As such organic compound, there can be mentioned, for example, alcohols such as methanol, ethanol and isopropanol, ethers such as tetrahydrofuran and dioxane, and ketones such as acetone and methylethyl ketone. Also water can be use for the same purpose. As the non-solvent used for the coagulating bath, water is ordinarily used, but organic solvents having a coagulation value lower than 100, such as alcohols, e.g., methanol, ethanol and isopropyl alcohol, can be used. The term coagulation value used herein indicates the amount (parts by weight) of the non-solvent or mixed non-solvent necessary for rendering opaque a solution containing 1% by weight of the polymer, when the non-solvent or mixed non-solvent is gradually added to 100 parts by weight of said polymer solution.
A composite (laminate-type) semipermeable membrane can be prepared by forming an ultra-thin membrane of the polymer (II) of the present invention on a preformed porous membrane. In this case, the preformed porous membrane used should have on the surface thereof pores having a size of up to 1.mu., preferably up to 0.5.mu. and also it should not be dissolved in the solvent used in the casting solution.
As such porous membranes, there can be mentioned a porous polypropylene membrane (manufactured and marketed under the tradename "Juraguard.RTM." by Polyplastics K.K.), a porous polyphenylene-oxide membrane (manufactured and marketed under the tradename "Necleopore.RTM." by Nomura Microscience K.K.), and porous membranes composed of polysulfone, cellulose triacetate and other synthetic resins. The ultrathin membrane of the polymer (II) formed thereon has a thickness smaller than 3.mu., preferably smaller than 0.5.mu.. It can be a uniform (homogeneous) membrane or an asymmetric membrane comprised of a dense and compact surface skin layer and a porous underlying layer.
This composite semipermeable membrane is obtained by casting into a preformed porous membrane, such as those mentioned above, a solution containing 0.1 to 5% by weight, preferably 0.5 to 3% by weight, of the polymer (II) dissolved in a single solvent or mixed solvent by means of a glass rod or doctor blade, evaporating a part or all of the solvent, and dipping the coated porous membrane in a coagulating bath as mentioned above.
In the production of the polymer (II), 15 to 90%, preferably 20 to 85%, of the hydroxyl groups of the polymer (I) should be transformed to sulfuric acid groups or salts thereof with an alkali metal, ammonia or nitrogen-containing basic organic compound.
Thus, the polymer (II) has the formula ##STR7## wherein R, R', X, l, m, p and n have the same meanings as defined above, and wherein from 15 to 90% of said D groups are sulfuric acid groups or salts thereof with an alkali metal, ammonium or nitrogen-containing basic compound and the balance of said D groups are hydroxyl groups.
The present inventors have found that the polymer (II) is remarkably affected in respect of its solubility, film-forming property and resistance to hydrolysis owing to the introduction of the sulfuric acid groups or salt thereof with an alkali metal, ammonium or nitrogen-containing basic organic compound, in place of the hydroxy groups of the polymer (I). The polymer (II) having only sulfuric acid groups can be formed into films. However, it is somewhat susceptible to hydrolysis in the procedure used for the preservation and preparation of the film. It has been examined how the polymer (II) salt with sodium, ammonium, dimethylamine or pyridine is soluble in various solvents as listed in Table 1. When the asymmetric film is prepared, the selection of a solvent to be used is significant. In general, there can be used a larger number of solvents when the polymer (II) salt is a salt of a nitrogen-containing basic organic compound such as pyridine and dimethylamine, than can be used with ammonium and sodium salts.
In addition, the polymer (II) salt with a nitrogen-containing basic organic compound has a smaller rate of hydrolysis, according as the pKa value of the nitrogen-containing basic organic compound is greater. The hydrolysis is measured by allowing the polymer, in the form of flakes, to stand at 60.degree. C., at 100% relative humidity for 20 days and determining the remaining amount of substitution. The results are illustrated in the drawing.